U846 - xorns and pits

Change pit behavior to always mark a non-flying poly'd player as trapped in
a pit, even when Passes_walls is set.  This allows players polymorphed into
xorns to descend into pits and pick things up.  In this case, any attempt
to move out of the pit now takes a turn but always succeeds.  Also fixed
a related bug (w/o ID) that a player could become trapped as if in a pit
when leaving xorn form because u.utrap wasn't checked, and simplified the
code since the extra Passes_walls checks are no longer needed.  Also
updated code in sit.c that duplicated uteetering_at_seen_pit.

src/sit.c

@@ -65,9 +65,7 @@ dosit()
 
 	if (OBJ_AT(u.ux, u.uy) && 
 	   /* ensure we're not standing on the precipice */
-	   !((!u.utrap || u.utraptype != TT_PIT) &&
-	     (trap && trap->tseen &&
-	     (trap->ttyp==PIT || trap->ttyp==SPIKED_PIT)))) {
+	   !uteetering_at_seen_pit(trap)) {
 	    register struct obj *obj;
 
 	    obj = level.objects[u.ux][u.uy];